The present invention relates to an exposure apparatus and an exposure method, and more particularly, to an exposure apparatus and an exposure method used in a lithography process for fabricating semiconductor integrated circuits, liquid crystal display device, thin film magnetic heads, or the like.
To fabricate devices, such as a semiconductor integrated circuit, a liquid crystal display device, and a thin film magnetic head, optical lithography is generally employed to form a circuit pattern on a substrate. During optical lithography, a exposure operation is performed, for example, in the following manner. A mask (original plate), on which a circuit pattern is formed, is arranged in an exposure apparatus and illuminated by an exposure light from a light source. The illuminated pattern is projected and the pattern of the mask is transferred onto the substrate, to which the photoresist has been applied, via an optical projection system.
A step and repeat (one-shot exposure) process is an example of such projection transcription. In the step and repeat process, shot areas, defined on the substrate, are moved in a stepped manner (stepping) and the circuit pattern is repeatedly exposed and transferred onto each of the shot areas. A step and scan (scanning exposure) process, in which the mask is moved together with the substrate while repeatedly exposing and transferring the circuit pattern onto the each of the shot areas, is also known.
An exposure apparatus, which projects and transfers a circuit pattern of a reticle onto a wafer, to fabricate a semiconductor integrated circuit will now be discussed.
In the exposure apparatus, the reticle and the wafer are arranged respectively on a reticle stage and a wafer stage. To properly project a pattern image from the reticle to the wafer or onto a circuit pattern already formed on the wafer, fine adjustments to the relative position and direction (rotation) of the reticle and the wafer must be made. In this case, the required position measurement, and subsequently performed position correction, or the required positioning, which includes both position measurement and position correction, are referred to as alignment.
The alignment includes reticle alignment and wafer alignment. The exposure apparatus has a reticle alignment sensor for measuring the position of a reticle alignment mark, which is formed together with the circuit pattern on the surface of the reticle. A wafer alignment sensor, which measures the position of a wafer alignment mark formed together with a circuit pattern on the surface of the wafer, is also provided. An alignment reference plate used to measure the relative position between the projected image of the reticle pattern and each of the shot areas on the wafer (including a base line amount of the wafer alignment sensor) is provided on the wafer stage. The reference plate has a reference mark that is used to perform position measurement with the two alignment sensors.
During reticle alignment, for example, the reticle alignment mark on the reticle surface is projected onto the reference plate, and the positional relationship between the reference mark and the reticle alignment mark is measured by a reticle alignment sensor. During wafer alignment, for example, the position of the reference mark and the position of the wafer alignment mark of at least three, for example, five to ten, shot areas, are measured using the wafer alignment sensor. The base line amount is then determined from the detection result of the reticle alignment sensor and the reference mark detection result of the wafer alignment sensor. Further, based on the wafer alignment detection result of the wafer alignment sensor, the position of each shot area on the wafer to which the reticle pattern is to be transferred is calculated as described in Japanese Unexamined Patent Publication No. 61-44429 and the corresponding U.S. Pat. No. 4,780,617. The information of the relative position between the reticle pattern projection image and each shot area obtained in this manner is used to move the reticle stage and the wafer stage so that the positions of the reticle and the wafer are matched in a two dimensional manner. When each wafer undergoes exposure for the first time, a circuit pattern and a wafer alignment mark are not yet formed on the wafer surface. Thus, the above wafer alignment is not performed. Further, when performing a TTR (Trough The Reticle) process, during which the wafer alignment sensor detects the reticle alignment mark and the wafer alignment mark, the detection of the reference mark, that is, the calculation of the base line amount is not necessary.
In a one shot exposure type exposure apparatus, at least one set of symmetrical reticle alignment marks are provided. In a scanning exposure type exposure apparatus, to reduce the influence of mark writing errors and compensate for mark measurement errors, multiple sets of the symmetrical marks are arranged and the positions of the multiple marks are measured.
Normally, an integrated circuit is formed from plural layers. The fabrication process includes the formation of a field oxide film, the dispersion of impurities, the formation of a gate oxide film, the formation of an insulating layer, and the formation of a wiring layer. The optical lithography process is repeatedly performed especially during the oxide layer formation process and the wiring layer formation process.
Since the pattern formed on the reticle differs for each process, the reticle must be changed when different processes are performed with the same exposure apparatus. After the reticle is changed and prior to exposure, reticle alignment must be performed to position the reticle at its predetermined position. Whenever exposure of the entire surface of a single wafer is completed, the wafer is exchanged with the next wafer, and prior to exposure, the new wafer must undergo wafer alignment to be positioned at its predetermined position.
Especially, during reticle alignment in the scanning exposure type exposure apparatus, the time for measuring the multiple sets of the reticle alignment marks is long. This results in a shortcoming in which the throughput of the exposure apparatus decreases significantly as the frequency of reticle exchanges increases.
Further, in recent years, to finely narrow the line widths of the circuit pattern in accordance with further integration of integrated circuits, for example, double exposure is performed to synthesize two types of patterns formed respectively on two reticles and form a pattern for a single layer of the wafer. Such double exposure inevitably increases the number of times the reticle is exchanged and further decreases the throughput of the exposure apparatus.
As described below, there are multiple types of double exposure.
(A) Exposure is performed by setting different optimal illumination conditions for a first reticle having, for example, an isolated line pattern, and for a second reticle having, for example, an L/S (line and space) pattern.
(B) Exposure is performed by arranging a first reticle and a second reticle, each having only an isolated line pattern, so that the isolated lines of the first reticle and the isolated lines of the second reticle are lined alternately to form an L/S pattern.
(C) A first reticle having an L/S pattern is first exposed, and then, a second reticle having a protection pattern and a thinning pattern is exposed. As a result, the L/S pattern corresponding to the protection pattern remains, and the L/S pattern corresponding to the thinning pattern is thinned thereby forming the isolated pattern.
(D) A first reticle having a phase shifter is first exposed, and then the residual areas at the peripheral portion of the phase shifter is exposed by a reticle and eliminated.
In types (A) and (B), the order of using the first and second reticles can be changed. However, in types (C) and (D), the second reticle must be exposed after the first reticle.
In this manner, during double exposure, the reticle must be exchanged at least once for every wafer and reticle alignment is necessary after exchanging the reticle. However, when a long time is required to exchange the reticle and perform reticle alignment, the throughput of the exposure apparatus decreases significantly. To solve this problem, the reticle exchange speed may be increased and the number of reticle alignment marks that are measured may be reduced. However, if the number of marks are simply reduced, the reticle position measurement accuracy and the mark writing error detection accuracy decreases. This results in another shortcoming and decreases the reticle alignment accuracy.
It is an object of the present invention to provide an exposure apparatus and an exposure method that prevents a decrease in throughput while achieving high mask alignment accuracy when performing an exposure process during which the mask is exchanged frequently.
A first aspect of the present invention provides an exposure apparatus for transferring a pattern formed on a plurality of masks onto at least one substrate. The masks are exchanged within a predetermined time period. The exposure apparatus includes a position sensor that measures relative position information of a plurality of reference marks arranged in correspondence with a plurality of measurement marks on the masks. A measurement controller is connected to the position sensor that controls the position sensor with a first position measurement mode that measures a plurality of first relative position information of the measurement marks and the reference marks and a second position measurement mode that measures second relative position information, the number of which is less than the first relative position information measured by the first position measurement mode. When the mask subsequent to an exchange matches the mask prior to the exchange during the predetermined time period, the second relative position information is measured by the second position measurement mode, the first relative position information associated with the mask subsequent to exchange is corrected using the second relative position information, and position information of the mask subsequent to the exchange is detected based on the corrected first relative position information.
In the first aspect, when the first relative position information of the plurality of reference marks and measurement marks is performed within a predetermined period, the second position measurement mode measures the second relative position information of some of the measurement marks and the reference marks. The first relative position information is corrected based on the second relative position information, and the position of the mask subsequent to exchange is detected based on the first relative position information.
In this manner, a first position measurement of a plurality of measurement marks and reference marks is performed when a mask exposes the first substrate of a lot. However, the number of measured marks may be decreased from the second exposure. This decreases the time required for mask alignment from the second exposure. In addition, the position of the mask subsequent to the exchange is detected by correcting the first relative position information using the second relative position information obtained through simple measurement that is performed from the second exposure. Accordingly, the mask alignment accuracy does not fall from the second exposure.
A second aspect of the present invention provides an exposure method for transferring a pattern formed on a plurality of masks onto a substrate. The masks are exchanged within a predetermined time period. The exposure method includes measuring first relative position information of a plurality of measurement marks arranged on the mask and a plurality of reference marks arranged in correspondence with the plurality of measurement marks. Then, the masks are exchanged for a plurality of times. Second relative position information of the measurement marks and the reference marks, the number of which is less than the first relative position information, is measured when the mask subsequent to an exchange matches one of the masks prior to the exchange during the predetermined time period. Next, the relative first position information related with the mask subsequent to the exchange is corrected using the second relative position information. The position of the mask subsequent to the exchange is then detected based on the corrected first relative position information.
The third aspect of the present invention provides an exposure method for transferring a pattern on at least one substrate in the same exposure apparatus using a plurality of masks that include a first mask and a second mask respectively having a first pattern and a second pattern. First, a plurality of marks formed on the first mask is detected to generate first position information. Next, the first pattern is transferred on a first substrate using the first position information, and the first mask is exchanged with the second mask. Then, the second pattern of the second mask is transferred to the first substrate or the different, second substrate. After exchanging the second mask with the first mask, some of the plurality of marks formed on the first mask is detected to generate second position information. The second position information and at least one piece of first position information are used to transfer the first pattern onto the first and second substrates and a different, third substrate.
In the third aspect, regardless of whether or not the substrate is exchanged, the first pattern of the first mask is transferred, the second pattern of the second mask is transferred, and then the first pattern is retransferred using at least one piece of the first position information and the second position information. Thus, during mask alignment when the first pattern is retransferred, the number of measured marks is decreased, and the time required for mask alignment is decreased.
In an exposure apparatus for a liquid crystal display device or the like, patterns of a plurality of masks are transferred in a continuous manner onto a plurality of shot areas of a substrate. A first pattern of a first mask may be transferred onto a first shot area, next, a second pattern of a second mask may be transferred onto a second shot area, and, then, the first pattern may be transferred again onto a third shot zone. In this case, the measurement marks for mask alignment during retranscription of a pattern on the same substrate is decreased. The time required for mask alignment is decreased not only during multiple exposure in which a plurality of circuit patterns is overlapped on a single shot area but also during exposure of a substrate for a liquid crystal display device in which a plurality of different circuit patterns are arranged along a plane.
A fourth aspect of the present invention provides an exposure method for transferring at least two patterns in an overlapped manner onto a plurality of substrates including a first substrate and a second substrate using a plurality of masks including a first mask and a second mask, which respectively have a first pattern and a second pattern. First, the first pattern of the first mask is transferred onto the first substrate, and the first mask is exchanged with the second mask. Next, the second pattern of the second mask is transferred onto the first substrate in a manner overlapping the first pattern, and the first substrate is exchanged with the second substrate. Then, the second pattern of the second mask is transferred onto the second substrate, and the second mask is exchanged with the first mask. Subsequently, the first pattern of the first mask is transferred onto the second substrate in a manner overlapping the second pattern.
In the fourth aspect, when the first substrate is exchanged with the second substrate, the second pattern is transferred onto the second substrate without exchanging the second mask. Then, the second mask is exchanged with the first mask, and the first pattern is transferred onto the second substrate in an overlapping manner. Thus, when performing multiple exposure on a plurality of substrates using a plurality of patterns, the number of times the mask is exchanged is decreased, and the number of mask alignment is decreased.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.